feat: add RFC 8707 resource indicator via resource config (#161)

* feat: add RFC 8707 resource indicator via resource config

`Config.Resource` is an alternative to `audience` for IdPs that implement
RFC 8707. `buildAuthURL` sends it as the `resource` parameter on the
authorization request, before `extraAuthParams` so an operator-configured
`resource` key there cannot override or duplicate it. `exchangeTokens`
repeats it on both the `authorization_code` and `refresh_token` grants
(RFC 8707 §2.2), so a refreshed token keeps the audience binding of the
original grant.

Only one resource is accepted. RFC 8707 permits a list, but no provider
that supports the parameter handles a list usefully: one rejects any
request naming more than one resource with HTTP 400 `invalid_target`;
node-oidc-provider accepts several and issues an opaque token bound to
none of them.

`Config.Validate()` requires an absolute URI of at most 256 characters
with no fragment, wildcard, or control characters. `http`/`https` values
go through `isValidSecureURL`, so HTTPS is required outside loopback;
other schemes (`urn:`) pass as-is.

When `resource` is set and `audience` is not, `audience` defaults to the
resource for access-token validation; `strictAudienceValidation` applies
the same way whichever of the two is set. That default is validation-only.
A new `TraefikOidc.explicitAudience` mirrors `Config.Audience` verbatim,
and `buildAuthURL` keys the non-standard `audience` request parameter (the
Auth0 custom-API pattern) on it rather than on `audience`, so a
resource-derived default is never sent as an unsolicited `audience`
parameter alongside `resource` — a combination that misbehaves against
providers such as Ory Hydra, which give `audience` a different meaning.
`audience_test.go` and `url_helpers_ultra_test.go` set `audience` on the
struct directly, bypassing `New()`, and now set `explicitAudience` too.

docs/CONFIGURATION.md carries only the reference-table row and a short
pointer; the new docs/RFC8707.md holds the request behaviour, the
single-resource rationale, the `resource`/`audience` relationship, and a
provider-support table from interop testing: node-oidc-provider supports
it; Zitadel accepts and silently ignores it (`oidc.AuthRequest` has no
`resource` field, confirmed against a live instance); Authentik returns
`invalid_target`; Keycloak has no support through 26.6; Ory Hydra/Fosite
uses its own `audience` parameter; Azure AD/Entra ID v2 is reported to
reject `resource` combined with v2-style `scope`; Auth0 and AWS Cognito
claims are unverified. README.md, docs/index.html, and .traefik.yml each
point at docs/RFC8707.md instead of repeating it, and the comments on
`Config.Resource`, in main.go, and in url_helpers.go defer to it as well.

rfc8707_resource_test.go covers the authorization-URL parameter (set,
unset, query-bearing value percent-encoded rather than merged,
`extraAuthParams` unable to override), both grants sending `resource` in
the form body, the `Config.Validate()` rules table, audience defaulting
through `New()`, and a regression pair: a resource-derived audience
default is not sent as `audience`, while an explicitly configured one
still is.

* fix: accept Resource in place of Audience for bearer-auth validation

`NewWithContext` rejected `EnableBearerAuth=true` whenever
`config.Audience` was empty, even though `t.audience` already falls back
to `config.Resource` in that case. The check now fails only when both
`Audience` and `Resource` are empty, and the error names both fields.
`TestStartupValidation_BearerAcceptsResourceInPlaceOfAudience` covers the
`Resource`-only configuration.

`docs/RFC8707.md` links the `audience` parameter note to
`docs/AUTH0_AUDIENCE_GUIDE.md` instead of restating it, and drops the
provider table's GitHub row and the interop-testing paragraph, which are
covered elsewhere.

types.go

@@ -168,14 +168,27 @@ type TraefikOidc struct {
 	// the same contract as providerURL, which updateMetadataEndpoints already
 	// reads unlocked. A manual override always wins over discovery; an empty
 	// string means use the discovered value.
-	configRevocationURL       string
-	configEndSessionURL       string
-	configIntrospectionURL    string
-	backchannelLogoutPath     string
-	frontchannelLogoutPath    string
-	scopesSupported           []string
-	scopes                    []string
-	extraAuthParams           map[string]string
+	configRevocationURL    string
+	configEndSessionURL    string
+	configIntrospectionURL string
+	backchannelLogoutPath  string
+	frontchannelLogoutPath string
+	scopesSupported        []string
+	scopes                 []string
+	extraAuthParams        map[string]string
+	// resource holds the RFC 8707 resource indicator from Config.Resource
+	// (Config.Validate() already guarantees it is either empty or a
+	// non-empty absolute URI). Sent as the `resource` param on the
+	// authorization request and on both the authorization_code and
+	// refresh_token grants.
+	resource string
+	// explicitAudience mirrors Config.Audience verbatim, with no
+	// resource- or clientID-based defaulting applied. audience (below) does
+	// carry that defaulting, for token-validation purposes; explicitAudience
+	// exists so buildAuthURL can decide whether to send the non-standard
+	// `audience` request parameter without that default leaking onto the
+	// wire.
+	explicitAudience          string
 	refreshGracePeriod        time.Duration
 	maxRefreshTokenAge        time.Duration
 	metadataMu                sync.RWMutex